Living spaces of residences and offices may carry airborne malodorous substances including sulfur compounds such as hydrogen sulfide and methyl mercaptan, nitrogen compounds such as ammonia, and other compounds such as fatty acid. To provide a comfortable living environment, it is desirable to treat the contaminated air for removal of the malodorous substances. In a medical environment, on the other place, antibiotic medication has induced inadvertent evolution of those bacteria having resistance against antibiotics. Among them, methicillin-resistant Staphylococcus aureus (MRSA), for example, has caused a serious problem of nosocomial (i.e., in-hospital) infection.
Accordingly, attention has been drawn toward the use of semiconductive photocatalyst for the purposes of processing the environment contaminated by bacteria and malodorous substances.
Since one of the inventors of the present invention and his coworker have reported their investigation on photoelectrolytic process of water (known as the "Honda and Fujishima effect") in a photoelectrochemical cell having a single-crystal semiconductor electrode of titanium dioxide (TiO.sub.2) in the form of rutile and a counter electrode of platinum (Nature, vol. 238(1972), 37-38), many researches have been carried out on the treatment of various media with a semiconductive photocatalyst.
To briefly set out the principle of the photocatalytic process in a photoelectrochemical cell with reference to FIG. 1 of the accompanying drawings, when a semiconductor photocatalyst is illuminated and is caused to absorb the light energy (h.nu.) higher than the band gap energy (Eg) of the semiconductor, the electrons in the valance band are photoexcited and raised into the conduction band to produce electron-hole pairs (e.sup.- -h.sup.+) at the surface layer of the semiconductor.
h.nu..fwdarw.e.sup.- +h.sup.+
In order for the thus generated electrons and holes to contribute in the photoelectrolysis of water, they must be moved, respectively, to separate oxidation and reduction sites that are spaced from each other (charge separation). Otherwise, the electrons and holes would recombine and convert into thermal energy without contributing in the redox process.
In a photoelectrochemical cell wherein a semiconductor-electrolyte junction is present, charge separation is carried out in the following manner. Thus, as the semiconductor is brought into contact with the electrolyte, transfer of charges occurs until the Fermi levels in both phases become equal. As a result, in the n-type semiconductor such as TiO.sub.2, the semiconductor surface is positively charged. The electric field developed by the charges causes a bending of the bands in the space charge region near the semiconductor surface as shown in FIG. 1. The electrons in the conduction band and the holes in the valance band are separated by the band bending, with the electrons e.sup.- moving to the bulk and the holes h+ moving to the surface of the semiconductor. The holes h.sup.+ moved to the semiconductor surface oxidize water to generate hydrogen EQU 2h.sup.+ +H.sub.2 O.fwdarw.1/2O.sub.2 +2h.sup.+
whereas the electrons e.sup.- transferred via the lead wire to the metallic counter electrode reduce water to produce hydrogen EQU 2e.sup.- +2H.sup.+ .fwdarw.H.sub.2
Since the success of the photoelectrolytic process of water in the photoelectrochemical cell has been published, A. J. Bard reported that, not only the photoelectrochemical cell having a single-crystal semiconductor electrode, platinized TiO.sub.2 powders also function as the photoelectrochemical cell (Journal of Photochemistry, 10(1979), 59-75). A. J. Bard considers that a platinized TiO.sub.2 particle functions as a short-circuited photoelectrochemical cell.
Thereafter, it has been found that even bare TiO.sub.2 particles present a photocatalytic effect and various investigations and researches have been made on the photocatalytic decomposition of ammonia, carboxylic acid, phenol and other compounds (e.g., H. Kawaguchi, Environmental Technology Letters, vol. 5, pp 471-474).
In this regard, it is believed that, in the photocatalytic decomposition of compounds, holes h.sup.+ and electrons e.sup.- generated by photoexcitation of semiconductor photocatalyst serve to oxidize and reduce surface hydroxyl group and surface oxygen, respectively, to generate OH radical (.multidot.OH ) and superoxide ion (O.sub.2.sup.-) EQU OH.sup.- +h.sup.+ .fwdarw..multidot.OH EQU O.sub.2 +e.sup.- .fwdarw.O.sub.2.sup.-
These species are highly active and induce redox process of the compounds. It is considered that photodecomposition of a compound is a multiple electron process. Thus, the original species is transformed through a plurality of intermediates into final products.
Turning to the prior art, it has been considered that, to photoexcite a photocatalyst to provoke a photocatalytic process, it is desirable to use ultraviolet radiations of a high light energy and to irradiate them at as high a light intensity as possible. For example, Japanese Patent Kokai Publication No. 2-280818 proposes a process of deodorizing air wherein UV radiations having a wave length of 250 nm are irradiated at an intensity greater than 2 mW/cm.sup.2 to excite a photocatalyst. Similarly, Japanese Patent Kokai Publication No. 63-267867 discloses a deodorizer device wherein UV radiations having a wave length of 250 nm issued from a germicidal lamp are irradiated on the photocatalyst. It will be noted, however, that UV radiation having such a short wave length of 250 nm is harmful to human bodies so that, under UV irradiation, it is necessary to wear protective glasses. Therefore, the prior art process is not directly applicable to a living environment so that the living spaces must be shielded against the UV light source. Furthermore, to strengthen the UV intensity, the light source must be located sufficiently close to the photocatalyst. This limits the surface area of irradiation. Another disadvantage is that expensive germicidal lamps made of quartz glass tube permeable to UV light of 250 nm wave length are required.
In Japanese Patent Kokai Publication No. 4-307066, there is disclosed a deodorizer system wherein UV radiation having a wave length of less than 410 nm is illuminated on a panel coated with photocatalyst. This system is designed such that the light source is shielded from the indoor space because the intensity of UV light is so high. Consequently, the system cannot be applied to sterilization of an exposed surface, such as the interior wall of a care room of a hospital, which may be accessed by people. Moreover, a special purpose light source for photoexciting the photocatalyst must additionally be provided.